In most human subjects, the surface of the cornea is a good approximation to a portion of a sphere. However, in some subjects the cornea deviates substantially from the ideal sphere and such corneas are said to be irregular. Irregular corneas may be a result of surgery or grafting, or may arise from natural ectasia (a thinning of the cornea that results in some areas being steeper than others), such as in keratoconus, which affects at least 1 in 2000 of the population.
Subjects with irregular shaped corneas will suffer impaired vision due to the irregular astigmatism and optical aberrations arising from the corneal distortion, quite apart from any other vision defects they may also have. Traditionally, such visual defects in subjects with irregular corneas have been corrected, or at least ameliorated, using rigid contact lenses which act as a replacement, regular corneal shape.
Fitting rigid contact lenses for subjects with irregular corneas can be greatly facilitated by the conventional technique of instilling a fluorescent orange dye (fluorescein) into the eye. The dye fluoresces green when exposed to blue light. The dye is taken up by the tear layer and so, under suitable illumination, the tears appear green in areas where they pool (e.g. in places where the contact lens does not fit in alignment with the surface of the cornea) and are a dark purple colour where the tear layer is restricted (e.g. where the contact lens is in contact with the corneal surface).
Whilst the use of rigid lenses is still the more common practice, in recent years, soft contact lenses have been used to correct vision defects in subject with irregular corneas. Soft contact lenses are typically made from hydrogels and have a high water content and higher oxygen permeability compared to rigid lenses. Accordingly they are more comfortable to wear. However, it is not possible to use the fluorescein technique to assist in fitting soft contact lenses because, inter alia, soft contact lenses drape more to the corneal shape resulting in a uniform tear film. Due to fluid take up of soft lenses, high molecular weight fluorescein derivatives have to be used with these lenses but even so, very little information as to fit can be deduced from the appearance of the fluorescing tear film. Accordingly, there is a high degree of skill required to ensure a soft contact lens achieves a good fit on an irregular cornea, and a significant amount of trial and error is required. It is an object of the present invention to provide an improved method of fitting a soft contact lens for a subject with an irregular cornea. In particular the method aims to optimise the efficiency of the fitting process by reducing the number of test fitting lenses required to be used and/or to reduce the amount of practitioner time to complete the fitting process.